Allen-Bradley 1756-CNB/E, User Manual

Page: 82 / 186

Publication 1756-UM523F-EN-P - December 2006
82 Configure and Program the Controller
Maintain Data Integrity
During a Switchover
The redundancy system guarantees a bumpless switchover for any
logic in the highest priority task. In some cases, a switchover may
make lower priority tasks repeat part of their scan. This has to do with
how data crossloads from the primary controller to the secondary
controller.
As the primary controller executes its logic, it updates the secondary
controller at the end of every program.
Switchover Overview
A. This data is sent to the secondary controller:
•
Data from the program in the higher priority task.
•
Data from the first part of the program in the lower priority
task.
B. Execution returns to the program in the lower priority task.
C. Data from the second part of the program in the lower priority
task is sent to the secondary controller.
When a switchover interrupts the execution of the primary controller,
the secondary controller reexecutes an interrupted program from the
beginning of the program.
Interruption by the Program
in a Higher Priority Task
Start of the Program in the Lower Priority
Task (for example, the continuous task)
Primary Controller
End of the Program in a
Higher Priority Task
A.
End of the Program in the Lower
Priority Task
C.
B.